On hppa the Instruction Address Offset Queue (IAOQ) registers specifies
the next to-be-executed instructions addresses. Each generated TB writes those
registers at least once, so those registers are used heavily in generated
code.
Looking at the generated assembly, for a x86-64 host this code
to write the address $0x7ffe826f into iaoq_f is generated:
0x7f73e8000184: c7 85 d4 01 00 00 6f 82 movl $0x7ffe826f, 0x1d4(%rbp)
0x7f73e800018c: fe 7f
0x7f73e800018e: c7 85 d8 01 00 00 73 82 movl $0x7ffe8273, 0x1d8(%rbp)
0x7f73e8000196: fe 7f
With the trivial change, by moving the variables iaoq_f and iaoq_b to
the top of struct CPUArchState, the offset to %rbp is reduced (from
0x1d4 to 0), which allows the x86-64 tcg to generate 3 bytes less of
generated code per move instruction:
0x7fc1e800018c: c7 45 00 6f 82 fe 7f movl $0x7ffe826f, (%rbp)
0x7fc1e8000193: c7 45 04 73 82 fe 7f movl $0x7ffe8273, 4(%rbp)
Overall this is a reduction of generated code (not a reduction of
number of instructions).
A test run with checks the generated code size by running "/bin/ls"
with qemu-user shows that the code size shrinks from 1616767 to 1569273
bytes, which is ~97% of the former size.
Signed-off-by: Helge Deller <deller@gmx.de>